Conventional automotive evaporative systems include a carbon canister communicating with a fuel tank to adsorb fuel vapors from the fuel tank. The carbon canister adsorbs the fuel vapor until it is saturated, at which time, the fuel vapor is desorbed from the carbon canister by drawing fresh air therethrough. Such a system is shown in FIG. 1. System 10 includes fuel tank 12 coupled to carbon canister 14 and engine 16 via vapor purge lines 17 and 24, respectively. Fuel vapor from tank 12 flows through line 17 into canister 14, where the fuel is adsorbed onto the carbon. Fresh air is then emitted through vent port 18 to atmosphere. When the canister becomes saturated with fuel, engine controller 19 command valves 20 to open so that the fuel may be desorbed from the carbon and flow to engine 16 via purge line 24.
Occasionally, it may be necessary to purge the canister when both the canister is full and a large vapor volume exists in the fuel tank. Thus, upon purging, in the system described with reference to FIG. 1, vapor is drawn from both the canister and the engine. As a result, the large vapor volume flowing directly from the tank to the engine may cause the engine to temporary run in an undesirably rich condition. To prevent this, a relatively small carbon canister 26, typically termed a buffer canister, is disposed between the fuel tank and the engine. This buffer canister 26, due to its relatively small size, quickly saturates such that the vapors flowing to the engine may break through the carbon bed to be consumed by the engine. The effect of the buffer canister is to reduce any large hydrocarbon or fuel vapor spikes going to the engine to prevent the over rich condition. In other words, the buffer canister acts to dampen any fuel vapor spikes typically flowing directly from the fuel tank to the engine.
The disadvantage with this approach is primarily due to the fact that a secondary canister must be utilized in the system. This creates added expense due to couplings, vapor lines, associated hardware and general system complexity. To overcome these disadvantages, some systems utilize a vapor purge line flowing directly from the tank to the primary carbon canister, with the purge line being embedded into the carbon bed. Such a system is depicted in FIG. 2. In this system, when fuel vapor from the fuel tank 12 is to be purged directly into engine 16, the fuel vapor must at least go through a portion of the primary carbon canister, shown at bracket 28. Thus, a portion of the canister acts to buffer any hydrocarbon spikes from the fuel tank.
The inventors of the present invention have found certain disadvantages with the system described in FIG. 2. For example, in order to utilize a portion of the primary canister as a buffer, fuel vapor line 17 must necessarily penetrate into the carbon bed. Because of this, manufacturing issues arise in that the vapor purge line must be sealed in a manner so as to prevent leakage between the line and the atmosphere at the intersection with the primary canister. In addition, the purge line must contain a screen or filter to prevent the carbon from dislodging from the canister. Furthermore, the amount of penetration is determined on a vehicle line basis. Thus, a relatively small engine may require a certain volume for the buffer whereas a relatively large engine may require a different volume. This fact requires unique manufacturing tooling to precisely locate the depth of the fuel tank purge line within the carbon canister.